强制转换为兼容返回类型的并集是否满足函数指针的兼容性标准？

为了解释为什么我要这样做，我的环境的约束要求我使用自动生成的代码。生成的代码非常相似，我想调用一批基本相同的函数。我需要使用符合C89或C99的解决方案。

通过阅读规范，以下内容似乎是合法的代码，但是我不确定是否要强制转换函数指针以返回联合类型。谁能指出这是否合法或违反了规范的哪一部分？

#include <stdio.h>
#include <stdlib.h>

/* Automagically generated types */

struct A_return_type {
    int index;
    unsigned int options;
};

struct A_return_type *A_function(int x) {
    struct A_return_type *A_return = malloc(sizeof(*A_return));
    A_return->index = x;
    A_return->options = 0xA;
    return A_return;
}

struct B_return_type {
    int index;
    unsigned int options;
};

struct B_return_type *B_function(int x) {
    struct B_return_type *B_return = malloc(sizeof(*B_return));
    B_return->index = x;
    B_return->options = 0xB;
    return B_return;
}

struct C_return_type {
    int index;
    unsigned int options;
};

struct C_return_type *C_function(int x) {
    struct C_return_type *C_return = malloc(sizeof(*C_return));
    C_return->index = x;
    C_return->options = 0xC;
    return C_return;
}

/* End generated types */

int main(int argc, char *argv[]) {
    /*--------------------------------------------------------------
    All of the generated methods take the same arguments and return
    structs with the same members in the same order. It is permitted
    to inspect the common initial part of any structs in a union,
    per C89 3.3.2.3 p5.
    --------------------------------------------------------------*/
    union return_types {
        struct {
            int index;
            unsigned int options;
        } common_return;
        struct A_return_type A_return;
        struct B_return_type B_return;
        struct C_return_type C_return;
    };

    /*----------------------------------------------------------
    Function pointers are compatible if their return types and
    parameter lists are compatible per C89 3.5.4.3 p9.
    ----------------------------------------------------------*/
    typedef union return_types *(*generated_function)(int);

    generated_function function_array[] = {
          (generated_function)A_function
        , (generated_function)B_function
        , (generated_function)C_function
    };

    for(int i = 0; i < sizeof(function_array)/sizeof(function_array[0]); ++i) {
        printf("%x\n", function_array[i](0)->common_return.options);
    }
}